The government postponed the hugely controversial decision on whether to build a third runway at Heathrow yesterday, sparking speculation that cabinet divisions on the environmental impact of the third runway had persuaded Gordon Brown of the need for delay.

Backbench Labour MPs, the opposition and environmentalists were all caught off-guard by transport secretary Geoff Hoon's announcement yesterday that a decision would be delayed until January 2009, 12 months after the initial consultation wound up. It is thought that the energy and climate change minister, Ed Miliband - a key ally of the prime minister for the past decade - persuaded him of the need for more consideration.

A decision had been expected in the next few days, leaving little parliamentary time for debate before Christmas. The government supported Heathrow expansion in its aviation white paper in 2003 and backed a third runway to that end, while Conservatives, Liberal Democrats and some members of the cabinet are thought to oppose it.

Hoon said yesterday he wanted time to give consideration to the 70,000 responses received when the government held a consultation on the issue.

Opponents of the third runway said division at cabinet level on the environmental wisdom of the decision was making the government rethink. The government's position had also been exposed by the Conservative party's decision to oppose the plans, saying they would fund high-speed rail links instead.

In recent weeks Gordon Brown has held private meetings in No 10 with Labour backbenchers, including a ministerial aide and a vice-chair of the party, in an attempt to assuage their fears. One Labour backbencher involved in the campaign said: "[Lord] Mandelson [the business secretary] is key to this decision - his two priorities are getting re-elected and keeping big businesses onside and in this instance the two are in conflict. And that's why maybe he needs more time." The Conservative MP for Putney, Justine Greening, called the delay a classic example of what her party have cast as Brown's "dithering". London's mayor, Boris Johnson, has said he favours a new airport in the Thames estuary.

Friends of the Earth thought the inauguration in the last week of a new climate change committee to hold the government's sustainability record to account had changed matters. FoE campaigner Richard Dyer said: "Hopefully this shows that ministers are beginning to wake up to massive environmental consequences of allowing airports to expand. Under the Climate Change Act, which was passed last month, the government is legally required to slash UK greenhouse gas emissions by 80% by 2050 ... but building new runways will cause a huge increase in aviation emissions that will make it all but impossible to meet our targets for tackling climate change."

However, the chair of the committee, Lord Turner, said recently that he thought airport expansion was consistent with the sustainability agenda.

Though Hoon has been only been transport secretary for two months, he has been happy to make controversial decisions. Only a few weeks into his job he gave permission for Stansted airport to increase flight capacity by 10%, overruling a local authority which banned the expansion on environmental grounds.

He said: "I know there are strong views across a range of interests. I will ensure that I give proper consideration to the evidence before me and will therefore take more time before making an announcement to the house, in January 2009."

A spokesman for Brown said the decision had "enormous" national and regional implications. "The delay is a reflection of the fact that this is an important decision," he added.

Liberal Democrat MP Norman Baker said: "Let's hope this is a belated sign that the government is finally paying attention to the massed ranks of the public and the Commons benches ... However, if this delay is just to wait for the aviation industry to rewrite its proposals in a way that suits it, then this is no cause to celebrate."

Business lobbyists expressed concern at the delay.

Ed Anderson, chairman of the Airport Operators Association, said: "That month will not change the fundamental fact that Heathrow's expansion is urgent and necessary for the future of the country, for London's standing as a world-class city, and for our regional economies."